Method and apparatus for controlling the thickness distribution of an interference filter

ABSTRACT

The thickness distribution of a vapor deposited layer such as an interference filter having deposited on a moving substrate such as a glass faceplate for a projection television tube, is controlled along an axis in the direction of travel of the substrate by employing at least one rotatable dodger to partially shield the substrate as it passes behind the dodger during deposition.

This is a division of application Ser. No. 07/340,835, filed Apr. 20,1989, now U.S. Pat. No. 4,942,063.

BACKGROUND OF THE INVENTION

This invention relates to multilayer interference filters formed byvapor deposition onto a moving substrate, and more particularly relatesto a method and apparatus for controlling the thickness distribution ofthe filter layers along the direction of movement.

U.S. Pat. No. 4,683,398 describes a cathode ray tube for projectioncolor television having a multilayer interference filter between theglass face panel and the luminescent screen. Such a filter results,among other things, in significantly greater brightness of theluminescent output of the tube.

Such interference filters are typically composed of alternating layersof materials having a high and a low index of refraction, respectively.The layers are preferably formed by vapor deposition onto the innersurface of the glass face plate.

Mass production of such filters is carried out in a vacuum chambercontaining the source materials to be evaporated, means for heating thesource materials, and a dome-shaped fixture adapted for holding amultiplicity of glass face plates. The plates are arranged in rows, eachrow forming a circle around the dome, so that each plate isapproximately equidistant with the other plates from the sourcematerials.

The dome is rotated during deposition, not only to promote uniformdistribution of the deposited material on the plates, but also to passthe plates behind one or more stationary dodgers located between thesource materials and the plates. These dodgers are designed to result inincreasing thickness of the layers toward the edges of the plates, whichhas been shown to result in even greater increases in brightness thancan be obtained for a uniform thickness distribution.

Ideally, the thickness of the layers should increase in all directionsfrom the center of the plate. Unfortunately, however, such anarrangement as described above can only be used to control thicknessdistribution in a direction normal to the direction of movement of theplates.

A method is known by which a radial thickness distribution can beachieved. This so called "planetary" method involves rotating each plateabout an axis normal to the surface of the plate, while the plate passesbehind the dodger as described above. However, in addition to adding tothe complexity of the apparatus, the rotating plates requiresignificantly more space in the dome than do stationary plates Thus, thethroughput of the apparatus is significantly reduced Also, in the caseof skirted panels, the planetary drive is adequate only for circularlysymmetric plates or "discs", and not for rectangular plates.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to control the thicknessdistribution of a vapor deposited layer on a moving substrate in thedirection of travel of the substrate.

It is another object of the invention to control the thicknessdistribution in such direction without rotation of the substrate aboutan axis normal to the surface of the substrate.

In accordance with one aspect of the invention, a method of controllingthe thickness distribution of a vapor deposited layer on a movingsubstrate in the direction of travel of the substrate, comprises thesteps of: (a) providing at least one dodger between the source materialand the path of the substrate; and (b) rotating the dodger about an axisnormal to the direction of travel of the substrate and substantiallyparallel to the plane of the dome or substrate when the substrate is atleast partially behind the dodger, so that during rotation of the dodgerthe substrate is partially shielded from and partially exposed to thesource material.

In accordance with a preferred embodiment of the method, the rotatingdodger is used in conjunction with at least one fixed dodger employed tocontrol thickness distribution of the deposited layer in a directionnormal to he direction of travel of the substrate.

In accordance with another aspect of the invention, an apparatus isprovided for carrying out the above method, the apparatus comprising avacuum chamber containing at least one source material and means forheating the source material to vaporize it, means for moving a pluralityof substrates relative to the source material while maintaining thesubstrates in positions approximately equidistant from the sourcematerial, at least one dodger, rotatably mounted between the sourcematerial and the path of the substrates, the axis of rotating of thedodger being normal to the direction of travel of the substrates, andsubstantially parallel to the substrate plane and means for rotating thedodger when a substrate is at least partially behind the dodger, so thatduring rotation of the dodger the substrates are at least partiallyshielded from and partially exposed to the source material.

In a preferred embodiment of the apparatus, the means for moving thesubstrates comprises a rotatably mounted dome-shaped fixture, thefixture adapted for holding the substrates in one or more circular rowsand for moving the substrates in these circular rows above the sourcematerial. The rotatable dodger may be used in conjunction with at leastone fixed dodger mounted between the source material and the path of thesubstrates, the dodgers being spaced apart from one another so that'they shield the substrates at different times during the substrates'travel about the source material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of the positional relationshipbetween source material, substrates and fixed dodger in a prior artvapor deposition apparatus;

FIG. 2 is a diagrammatic representation of the relationship betweensubstrate and dodger as viewed from the source in the apparatus of FIG.1;

FIG. 3 is a diagrammatic representation of the relationship between asubstrate and a rotatable dodger as viewed from the source in accordancewith the invention;

FIG. 4 is an end view of a substrate and rotatable dodge and oneembodiment of a mechanical means for rotating the dodger synchronouslywith the passage of the substrate behind the dodger;

FIG. 5 is an end view similar to that of FIG. 4 showing anotherembodiment of mechanical means for rotating the dodger synchronouslywith the passage of the substrate;

FIG. 6 is a diagrammatic representation of a vapor deposition apparatusof the invention showing computer controlled means for rotating therotatable dodger;

FIG. 7 is a graph of thickness distribution of a layer deposited alongthe minor axis of a face panel in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the positional relationship between source material,substrates and fixed dodger in a prior art vapor deposition apparatus.Dome-shaped fixture 1 holds substrates 2 and 3 in positions which areapproximately equidistant from source materials 7 and 8. Fixture 1rotates about an axis 4. Fixed dodgers 5 and 6 shield substrates 2 and 3respectively from the path of vapor from the source materials each timethe rotating fixture brings the substrate behind the dodger.

In the application envisioned for the invention, the substrates areglass face panels for cathode ray tubes for projection television, andthe source materials are usually oxides of silicon and titanium ortantalum, which are deposited on the inner surfaces of the substrates inalternate layers. Oxidation is later completed to form an interferencefilter of alternating layers of SiO₂ TiO₂ or Ta₂ O₃ having low and highindices of refraction, respectively.

In this application, the ability to control the thickness distributionof the filter layers across the face plate enables the optimization ofcertain operating characteristics of the projection tubes such as lumenoutput to the projection optics.

A certain degree of control over thickness distribution is possibleusing fixed dodgers such as dodgers 5 and 6 shown in FIG. 1. FIG. 2shows the relationship between dodger 5 and substrate 3 as viewed fromthe position of the source materials. Substrate 3 is a glass face platewhich is rectangular in shape. The long axis of the face plate isconventionally referred to as the major axis which in this Figurecorresponds to the Y axis. The short axis is the minor axis and herecorresponds to the X axis, which represents the circular path of travelof the substrate caused by the rotation of fixture 1, as viewed in theplane of the circle.

Dodger 5 has a width W which varies as a function of distance Y from theX axis. As the substrate is moved behind the dodger, the substrate isshielded from the evaporation source and less material is deposited onthe substrate than would otherwise be the case. If the dodger hadapproximately straight sides parallel to the Y axis, all of thesubstrate would be shielded for the same proportion of the time and thedistribution of the deposited layer would not change. However, since thedodger width varies as a function of Y, the. substrate exposure timealso varies along the Y axis and the thickness of the deposit alsochanges.

It is not possible to control thickness along the minor axis with staticdodgers of any geometry. In accordance with the invention, thicknessalong the minor axis is controlled using a rotating dodger. FIG. 3 showsan arrangement as viewed from the position of the source materials inwhich dodger 30 is mounted between the source and the substrate 2 torotate along its longitudinal axis, parallel to the Y axis of thesubstrate. While FIG. 3 shows the dodger as having a constant; width, W,the width can vary to fine tune the final thickness distribution.

The dodger is normally in an open position, that is, normal to thesubstrate surface, alloWing essentially unobstructed access of the vaporfrom the source to the substrate. As shown in FIG. 3, as the leadingedge of substrate 2 passes by dodger 30, the dodger rotates to a closedposition (FIG. 3b) to shield the central part of the substrate behindthe dodger from the vapor. As the trailing edge of the substrate 30bapproaches the dodger, the dodger again rotates to an open position(FIG. 3c).

A typical mechanical arrangement for achieving the rotation of thedodger synchronously with the passage of the substrate is shown in FIG.4. Dodger 30 is attached to a cam 31 having four protruding fingers 31athrough d. Studs 32 and 33 protrude downward from fixture 40 As thefixture 40 carries substrate 2 past dodger 30, fingers of cam 31successively engage studs 32 and 33. Each time a cam finger engages astud, the dodger is rotated 90 degrees.

Another embodiment of a mechanical arrangement of rotating the dodger isshown in FIG. 5. In this arrangement, the dodger 50 is held in anormally open position by spring 51. As fixture 52 brings substrate 53past dodger 50, cam 54 engages stud 55 and rotates dodger 50 into aclosed position When stud 55 disengages cam 54, spring 51 rotates dodger50 back to an open position. Other arrangements will become apparent tothose skilled in the art.

By contouring the studs and the cams, the speed of opening and closingthe dodger can be controlled, thereby controlling the distributionprofile in the area between the center and the edge of the substrate.

A typical thickness distribution along the minor axis achieved by suchan arrangement is shown graphically in FIG. 7. The thickness of thedeposited layer is plotted in terms of percent deviatiOn frOm centerthickness along the Y axis, versus distance in inches along the minoraxis, here the X axis. Three curves are presented, all of whichrepresent thickness distributions obtained on 20 layer SiO₂ /TiO₂filtering in a 1100 mm vacuum coater containing 24 substrates in tworows. The collet rotated at 30 RPM and coating time was about 1.5minutes per layer for SiO₂ and 3 minutes per layer for TiO₂. Curve 1represents the thickness distribution obtained without the use of arotating dodger, while curves 2 and 3 represent the thicknessdistributions obtained when using two rotating dodgers 1" wide,mechanically driven with a system similar to the one pictured in FIG. 5.

A suitable apparatus 60 for carrying out the method of the invention isshown schematically in FIG. 6. The vacuum enclosure 61 contains therotating collet 62. The collet is supported by a shaft 73 and rotated bya motor 74 located outside the vacuum chamber 61. Collet 62 holds atleast one substrate 63 having a surface exposed to the interior of theenclosure. Rotating dodger 64 is mounted between substrate 63 andsources 69 and 70 The sources are heated to evaporation temperatures byseparate heaters 71 and 72, typically using electron heating means.

The apparatus includes means for rotating the dodger comprising motor 65connected to dodger 64 by drive coupling 67. The motor may be locatedinside or outside the vacuum chamber 61. Both the motor 65 and theheaters 71 and 72 are controlled by computer 68. One advantage of thisarrangement is that the dodger can be rotated at any one of a range ofspeeds selected by inputting appropriate commands into the computer.Thickness distribution can be controlled by controlling the rate atwhich the dodger is rotated, or in systems where many revolutions perlayer are made, by controlling the amount of time the dodger is heldopen or closed. Further advantages include a generally smoother rotationof the dodger than can be obtained using mechanical means and theability to automatically control the heaters in accordance with adesired schedule.

As will be appreciated, there is a limit to the amount of correction tothe thickness distribution of the deposited layer which can be obtainedfrom a single dodger. For the conditions described with reference toFIG. 7, it is seen that the use of a dodger in accordance with theinvention results in a maximum thickness at the edge of the substrateabout 1.7 percent greater than the thickness obtained without such adodger. Of course, for greater corrections than can be obtained with asingle dodger, multiple rotatable dodgers may be used.

Other means can be used to synchronize the motion of the dodger and themotion of the collet. The dodger can be driven by a belt, chain or geartrain from the collet, collet shaft or collet drive motor.

If it is desired that the layer thickness decrease from center to edgeof the substrate, the dodger can be normally closed instead of normallyopen, and rotate to the open position at the central region of thesubstrate, instead of close.

The rotatable dodger described herein can be used in conjunction with afixed dodger of the prior art. With such a combination, the thicknessdistribution can be made a nearly arbitrary function of X and Y bymaking one or more of the following adjustments:

(a) changing the shape of the fixed dodger;

(b) changing the width of the rotatable dodger along its length;

(c) changing the shape of the cams and studs that drive the dodger orotherwise controlling the rates at which the dodger opens and closes orthe amount of time the dodgers are held open or closed;

(d) changing the number of fixed and/or rotatable dodgers in the system.

The invention has been described in terms of a limited number ofembodiments. Other embodiments and variations of embodiments will becomeapparent to those skilled in the art, and these are intended to beencompassed within the scope of this description and the appendedclaims.

We claim:
 1. A vacuum evaporation apparatus for forming vapor depositedlayers from at least one source material on a plurality of movingsubstrates, the apparatus comprising a vacuum chamber, means within thechamber for heating the source material to vaporization temperature,means for moving the plurality of substrates in a path relative to thesource material while maintaining the substrates in positionsapproximately equidistant from the source material,characterized in thatat least one dodger is rotatably mounted in the vacuum chamber betweenthe source material location and the path of the substrates, the axis ofrotation of the dodger being normal to the direction of travel of thesubstrates and substantially parallel to the substrate, and in thatmeans are provided for rotating the dodger when each substrate is atleast partially behind the dodger, so that during rotation of the dodgerthe substrates are at least partially shielded from and partiallyexposed to the source material.
 2. The apparatus of claim 1 in which themeans for moving the substrates comprises a rotatably mounteddome-shaped fixture, the fixture adapted for holding the substrates inone or more circular rows and for moving the substrates in thesecircular rows above the source material.
 3. The apparatus of claim 1 inwhich at least one fixed dodger is mounted in the vacuum chamber betweenthe source material location and the path of the substrates, the dodgerbeing spaced apart from the rotatable dodger so that the dodgers shieldthe substrates at different times during the substrates' travel aboutthe source material.
 4. The apparatus of claim 1 in which the means forrotating the dodger comprises a cam attached to the dodger and at leastone stud attached to and extending from the means for moving thesubstrates, so that as the substrates move past the dodger, the studengages the cam and rotates the dodger.
 5. The apparatus of claim 1 inwhich the means for rotating the dodger comprises a motor, drive meansconnecting the dodger and the motor, and means for controlling themotor.
 6. The apparatus of claim 5 in which the motor is mounted insidethe vacuum chamber.
 7. The apparatus of claim 5 in which the motor isoutside the vacuum chamber.
 8. The apparatus of claim 5 in which themeans for controlling the motor also controls the means for heating thesource materials.
 9. The apparatus of claim 1 in which the rotatingdodger is driven by a melt, chain or gear train from the rotatingcollet, the collet shaft or the collet drive motor.